Rubber goods and polymer barrier structures have long been employed, commonly formed of natural rubber (latex rubber) films formed by dip molding. A variety of problems beset the natural rubber products, including the occurrence of dermatitis, allergenic reactions, and anaphalactic shock reactions among people who work with or are subjected to exposure to latex products. In addition, latex rubber goods, and particularly dip molded latex rubber goods, have a substantial porosity with dimensions on the general order of the size of virus particles; when stretched in use, the pore dimensions are enlarged. The barrier effectiveness of latex rubber films and sheets is rather less than optimum.
In recent times, these problems have led to the employment of synthetic polymer elastomers in the fabrication of such rubber goods. The results have been good in many cases, but at a considerable price. Such products are normally formed from far more expensive polymers, and from dipping solutions in organic solvents which are also expensive, both in terms of materials costs and in handling requirements for environmental protection and safety considerations. In addition, the problems of quality control and uniformity of such products have not been fully and satisfactorily resolved, as pin holes and porosity remain common. Such products are little improved and generally far more expensive to produce. The potential benefits of employing synthetic polymer elastomers are only partly realized.
Among the elastomers employed in such production have been the class of thermoplastic polyurethane elastomers and cross-linked polyurethane elastomers. Polyurethanes offer very attractive properties for rubber goods but these benefits have not been fully and cost effectively realized.
Difficulties in dip molding of thermoplastic polyurethane elastomers include the following specific problems:
Solvents for thermoplastic polyurethane elastomers are highly polar organic solvents, typically tetrahydrofuran (THF), dimethyl acetamide (DMAC), dimethyl formamide (DMF), methyl ethyl ketone (MEK) and methylene chloride (MC), used alone or in blends. The polymers are dissolved slowly with heat and/or mixing, typically at polymer concentrations of about 3 to 7 weight percent.
The viscosity of the solution is directly proportional to the polymer concentration in such solutions, and can be increased or decreased by decreasing or adding, respectively, the proportion of the solvent.
At high viscosity, the formation of continuous films of the polyurethane is assured, but high viscosity also results in the formation of thick films which have other difficulties, particularly in the drying stage, as discussed below.
If the polymer concentration is reduced by adding additional solvent, film continuity suffers, with increasing proportions of pinholes, discontinuities and other film defects.
Balancing polymer concentration against viscosity is a quite difficult aspect of polyurethane dip molding, and is often made more complex by the additional considerations discussed hereafter.
The solvents effective for thermoplastic polyurethane elastomers did molding solutions are very highly volatile. Since the solvent is to be removed by evaporation, such a property is often considered a desirable attribute but, in practice, the effective range of solvents have proved to be excessively volatile, and introduce additional problems.
In most cases, the rapid evaporation of the solvents results in the formation of a thin surface film or crust of gelled or solidified polymer, with a substantial proportion of the solvent entrained within the depth of the film. The entrained solvent is evaporated from the polymer at a very slow rate, even at relatively high drying temperatures, and complete removal of the solvent is rarely attained.
One result of solvent entrainment is the solvent plasticizing of the polymer, altering the polymer properties for which the thermoplastic polyurethane elastomers is employed. The entrained solvent reduces the Vicat softening temperature, reduces elongation at break and tensile strength, and reduces the cut and puncture resistance of the film. Other properties of the polymer film are degraded as well.
Rapid evaporation of the solvent also serves to prevent "leveling" or coalescence of the polymer during the film formation, resulting in an unwelcome population of pin holes and other discontinuities which limit the barrier film properties of the material.
It is common to dip mold in multiple dipped layers in the formation of barrier film materials and products to overcome the incidence of pinholes. When multiple layers are dipped sequentially, the thin surface film and high levels of entrained solvent in an earlier layer can result in blistering and bubbling between the layers, introducing another form of discontinuity in the film.
The entrainment of solvents can also result in blushing and bleeding of the solvent at the surface of the film over time; for many uses, such properties are inappropriate and limiting. For implantable uses within the body, such bleeding is intolerable.
The crusting over of the film and the entrainment of solvents imposes a requirement for excessively long and expensive drying to remove as much of the solvent from the molded product as possible.
When the thermoplastic polyurethane elastomers is cross-linkable or curable, the significant levels of solvent entrained in the film can retard or inhibit the post forming reactions, resulting in an inferior cure of the polymer film.
The combination of high viscosity and high volatility of the dipping solutions combine to constrain the nature and amounts of ancillary compounding ingredients that can be employed in the films molded by such techniques. Compounding additives typically increase the viscosity and reduce the flow properties of such solutions and limit the effectiveness of the dip molding operation as the amounts are increased.
The entrainment and subsequent bleeding of residual solvent from dip molded goods is and unacceptable property for medical implants and many rubber goods designed for employment within the body of patients during surgical procedures. These constraints have considerably limited the use of thermoplastic polyurethane elastomers in rubber goods intended for such uses.
The thermoplastic polyurethane elastomers dip molded rubber goods require more expensive materials that natural rubber latex film rubber goods, including the polymer and the solvents. Such thermoplastic polyurethane elastomer goods thus necessarily face a materials cost disadvantage. The highly desirable properties supposed to be available and the avoidance of the specific disadvantages of latex products would ordinarily offset the cost disadvantage for substantial numbers of uses. In practice, however, the thermoplastic polyurethane elastomers introduce other cost disadvantages in addition to the cost of materials.
The thermoplastic polyurethane elastomers are slow to dissolve in the solvents, and require additional processing equipment for forming the solutions. Latex forms of natural rubber do not require such operations. In addition, the efforts to remove the solvent from the dip molded films is quite time consuming. As a result, the dip molding operations are more labor intensive, capital equipment intensive, and production rate limited when the thermoplastic polyurethane elastomers are employed, compared to natural rubber latex.
As noted above, the balancing of the dip molding characteristics of the thermoplastic polyurethane elastomer solutions is quite demanding and difficult. It is common to see quality control losses in production of rubber goods from the thermoplastic polyurethane elastomers of 30 to 40% for demanding quality control specifications. Such losses stem from high levels of pin holes and other discontinuities, from excessively thick films, and other similar problems.
As a consequence, rubber goods molded of thermoplastic polyurethane elastomers have proved excessively expensive and have met with limited commercial success despite the considerable theoretical advantages to be gained by the employment of these materials.